1. Field of the Invention
This invention relates to a method for setting and controlling desired redox potentials in gases, particularly in hydrogencontaining protective gases which are used in sintering furnaces for oxide-ceramic bodies.
2. Description of the Prior Art
In many powder-metallurgical processes, which are normally carried out in high-temperature furnaces, reducing gas atmospheres are used for ensuring and setting the desired product quality. In almost all cases, these protective gases contain hydrogen gas as the reducing medium. The redox potential of hydrogen is determined, as seen thermodynamically, by the ratio of the hydrogen content to the water content according to the following formula: EQU .DELTA.G.sub.O.sbsb.2 =RT ln P.sub.O.sbsb.2 =2.DELTA.G.sup.O.sub.T +2RT P.sub.H.sbsb.2.sup.O /P.sub.H.sbsb.2
where
.DELTA.G is the free enthalpy; PA1 R gas constant; PA1 T absolute temperature; PA1 P.sub.O.sbsb.2 the partial pressure of oxygen; PA1 .DELTA.G.sup.O the free formation enthalpy for O.sub.2 under normal conditions (1 atm, 25.degree. C.).
From this it follows that the reducing power of the hydrogen-containing gas can be influenced by the admixture of water. However, this insight has heretofore been used only to the extent that the redox potential was thereby set only approximately to a defined value.
With the further development of powder metallurgy technology it was found that exact setting of the redox potential of the protective gas is of great importance for production of constant, high quality end products.
A problem, however, arose when variations and fluctuations in throughput occurred resulting in changes of the redox potentials of the protective gases and production of varying and inferior quality products. The product throughput is significant inasmuch as water is produced by reduction processes that occur.
In addition to assurance of constant product quality, the setting and controlling of the redox potential should desirably take place in an easily checked and documented manner, independently of the operating personnel.